A combustion unit of this type is known, for example, from the following reference: F. A. Zenz, Fluidization and Fluid Particle Systems, Pemm-Corp. Publications, vol. II, Draft 1989, pages 333-334. This publication deals with a so-called bubbling fluidized bed reactor. Due to its structure and flow properties. The reactor is, however, so inconvenient that there are no existing practical applications, especially where the reactor chamber should accommodate a particle separator.
Fluidized bed boilers based on circulation reaction technique, to which technique also the present invention belongs, are becoming increasingly popular in technical applications since they enable reduction of sulfuric and nitrogen oxide emissions to legitimate levels at very low costs. Especially with sulphur containing fuels the economy of the circulation reactor is excellent if the heat power of the plant is lower than 200 MW. In technical applications, the primary purpose of the combustion is the production of thermal energy which is further conducted to the heat transfer medium of the reactor chamber, which is usually water. Therefore, the walls of the reactor chamber usually contain a tubular structure formed by several parallel tubes and finlike units connecting together the tubes and forming a gas tight panel structure. Traditionally, particle separators and return system for solid material are structures placed separately from the circulation reactor chamber. The particle separator and the return system normally comprise an outer steel supporting structure and an inner ceramic layer which is to insulate the steel structure from the hot particle-gas suspension. The advantage in this kind of construction is the structural simplicity of the reactor and the particle separator. A great deal of experience has accumulated on its practicability in practical applications. The drawback in the traditional constructions is the requirement of the space since both the reactor chamber and particle separator are, in these constructions, have substantially equal main dimensions and they must be placed, for constructional reasons, far from each other. This brings about the drawback that the return system for the solid material becomes complicated in design because it must have a separate system for controlling the gas flow which system, in practice, is realized by a separate fluidized bed disposed in the return duct.